WatchAction: Clean up redundant getFormFields() and show()
* Make FormAction::getFormFields not abstract. In most cases this will just
be an empty array. This is in prep for RollbackAction as well.
* Remove redundant show() in WatchAction. This used to do custom stuff,
but after
77cdf1919 it does exactly the same as the parent FormAction::show.
* Don't add 'redirectparams' hidden field if there were no custom query parameters
(e.g. plain index.php?title=..&action=..)
Change-Id: Ia7f9bb0367c49a23179e9fefa9f529fa8aef8f52